EXPLAIN FORMAT=JSON: Better output

MDEV-6109 has added support for EXPLAIN=JSON. The basic structure is there, but the output can be improved for a number of cases. This is a task to list/work on them.

Very important

• JSON-ish output for ORDER BY/GROUP BY/ DISTINCT (MDEV-6995)
• Walk through what we currently produce and see if it all makes sense.

Not very important

• Subquery cache feature (ANALYZE should print cache's statistics)
• Better/systematic display of subquery types (Materialization with/without keys, single-row subquery, etc)
• Check how hash join is displayed. Could we do better?
• Better display for MRR scans (display them as special kinds of scans)
  • ANALYZE should display scan restarts/buffer refills for "Key/Rowid-ordered scan"
  • non-bka case also should display buffer refills.
• UNION/UNION ALL should display whether a temp table is used.

Minor

• Both us and MySQL print "ref": ["func"] . This made sense for tabular output but makes much less sense now.
• MariaDB doesn't print subqueries in the WHERE clause. However, it still prints item_in_optimizer:

  "(not(<in_optimizer>(t1.a,t1.a in (subquery#2))))"

  should this be removed?

Done

• are "expensive const" conditions printed anywhere